Telephone system



Aug. 15, 1939.

R. TAYLOR El AL 2,169,302

TELEPHONE SYSTEM Filed April 25, 1936 8 Sheets-Sheet 1 MTR Aug. 15, 1939. TAYLOR AL 2,169,302

TELEPHONE SYSTEI Filed April 25, 1936 8 Sheets-Sheet 2 6 i 72 04/45 A} if Afr-7):"

Ahg. 15, 1939. R, T LOR Er AL 2,169,302

TELEPHONE SYSTEI Filed April 25, 19356 8 Sheets-Sheet 4 rig! 0155 9 g of6 I 77 (7 FY? I ToPCZ TOPCS R sa2 91:2 902 sdz RsezR Aug. 15, 1939. R- TAYLQR Er AL 2,169,302

TELEPHONE SYSTEM Filed April 25, 1956 8 Sheets-Sheet 5 9 L l L PA 0 12 0..Z7... R8... 22

Pia/V410 72mm meaz 7270/1/45 54415 Arm Aug. 15, 1939. R TAYLOR ET AL 2,169,302

\ TELEPHONE SYSTEM Filed April 25, 1936 8 Sheets-Sheet 6 0 RI 0 0. R6 0 O Q4 o a R. TAYLOR ET AL 2,169,302

TELEPHON E SYS TEM Aug. 15, 1939.

Filed April 25, 1936 8 Sheets-Sheet 7 HM MA Aug 15, 1939.

R. TAYLOR ET AL 2,169,302

TELEPHONE SYSTEM Filed April 25, 1936 8 Sheets-Sheet 8 1 i I I frZ I 1 v! I T frG Mk 7 58 I Arrr.

Patented Aug. 15, 1939 UNITED STATES PATENT OFFICE TELEPHONE SYSTEM Application April 25;

1936, Serial No. 76,322

In Great Britain May 13, 1935 18 Claims.

The present invention relates to telephone systems and has for its object the provision of an improved finder switch arrangement for connecting a calling subscribers line to a first numerical switch. The invention may be considered as an improvement on the arrangement disclosed in our Patent No. 2,091,127, granted August 24, 1937, in which primary line finders are arranged so that those first taken into use are directly connected to first group selectors, while those last taken into use are arranged to be connected to first group selectors in a common group over secondary finders.

Since switches of this type are required to operate at high speed it is important that they shall be reliably stopped when the wipers encounter the calling line, and according to one feature of the invention this is secured by shunting the interrupter contacts so that the driving magnet is held energised.

According to another feature of the invention the test for a free last choice finder and its associated secondary group is made simultaneously so that under all traffic conditions dual or lost connections are avoided.

According to a further feature of the invention if the secondary group seized by a last choice primary finder cannot complete the call, for example due to all secondary finders in the group being busy, or to a fault, the primary finder will be released and another connected to a difierent secondary group will be taken into use.

A still further feature of the invention which is concerned with the routing of last choice finders is that should all such finders become engaged due to incident trafiic, further calls are routed to the first choice finders and the group is not closed until all first and last choice finders are engaged. As in the prior arrangement the primary finder switches in connection with which the present invention will be described are of the two-directional type adapted to operate in both directions of motion in a self-interrupted circuit. No release magnet is provided and the restoration of the switches to normal is effected by the intermittent operation of the secondary magnet, so that the switch wipers in the complete setting and restoration operation perform a socalled rectangular movement. Control of the primary and secondary finder switches is by means of common sets of apparatus associated with the switches over rotary allotter switches as in the prior arrangement.

The invention will be better understood from the following detailed description of one method of carrying it into effect, reference being had to the accompanying drawings comprising Figs. 1 to 8 which should be arranged as shown in Fig. 9 to form a complete circuit.

Referring first to the individual figures, Fig. 1 shows a subscriber's line circuit, primary control start relays which are associated with each 200- line group of subscribers, a thermionic valve VAA and relays RFB and OFB which together form a group busy detecting device for both first and last choice finders. Fig. 4 comprises the start relay set which has the function of starting up the control sets either separately or together depending upon the calling conditions. Three primary control sets P01, P02 and P03 are assumed to be fitted, full details of PC I being disclosed in Figs. 5 and 6. The strapping shown in Fig. 4 between terminals l0 and H allows for three primary control sets and their associated allotters which may be simultaneously started up if there are three or more simultaneous calls in the group but if only two control sets PCI and P02 are provided the strapping between terminals III and H is omitted and instead terminal in is strapped to terminal l2 and terminal H to terminal l3. Figs. 2 and 3 show full details of a line finder and the proper number of such finders required for each subscribers group are connected to the banks of the primary allotters such as PA. The finders are divided up into first choice and last choice finders and the arrangements are such that the last choice finders are not taken into use until all the first choice finders become busy. The necessary changes of connection according to the conditions under which the finders are to be used are made at the I. D. F. and there are also certain strapping changes to be made on the finders internally. Thus the chain-dotted connections shown are suitable for first choice finders which are directly connected to first selectors while the circle and dash connections apply to last choice finders which are connected with first selectors by way of secondary finders. Fig. 8 covers details of a secondary finder control set SCI in conjunction with a secondary start set SSI which can start up two control sets SCI and SC! while Fig. 7 gives details of the secondary allotter SA which has access to a group of secondary finders such as SFI. The secondary finders have access over the I. D. F. to last choice primary finders.

It will be understood that the secondary start and control groups are accessible in common from primary allotters serving different groups of lines and are provided in numbers sufiicient tocateri themaximumtrafiic on thesegroups also arranged that adjacent conbanks of the primary allotters exdifierent secondary control groups in increase the possible availability althmh it is conceivable that the same secondary control group may make more than one appear- LD. 1''. to a selector as leadisnotrequiredandthesllleadisstrapped over the I. D. F.-to the positive lead. The D lead when required is strapped through to the first selector and is utilised when a group of finders is arranged for serving mixed coin-box and ordinary subscribers. Normal post springs NPI are fitted in sucha case and are arranged to operate on the level upon which the coin-box sub scribers are accommodated; this i is only wired on first choice finders. The negative, positive, P and 81! leads oi last choice finders are connected over the I. D. F. to the bank multiple of the secondary finders while the 88 lead is connected across to the secondary start relay sets.

For the purpose description it will first be assumed that the finder shown is one o! a first choice group and only the chain dotted connections shown therefore will apply.

Referring now to Fig. 1, when the subscriber whose line circuit is shown removes his receiver, the loop placed across the incoming speaking leads operates relay L. Thereupon armature it marks the position of the calling line in the rotary bank multiple oi. the finder, armature it extends a marking battery to the finder vertical bank multiple and operates the level start relay SE and armature l2 disconnects the winding of relay K lrom the P lead extending to the final selector multiple and applies earth thereto in order to busy the line to incoming calls.

Assuming for convenience of description that start relay 8A is energised rather than relay SE, a circuit is completed from armature sol, Fig. 4, to operate the start relay ST, Pig. 5, in the primary control set PCi .by way of rectifier MRE in the conductive direction. It should be explained that the primary allotter PA is arranged to be preselecting so that under conditions of light trafiic its wipers will already be standing in engagement with a tree finder. Assuming this to be the case, armatures stl, st! and sit prepare a circuit for a delayed alarm change-over device tobe described later while armature stl completes a circuit from earth, left-hand winding of relay LK, armature rlzl, wiper R2 0! pri mary allotter PA, chain-dotted connection, 01!- normal springs N2, armature ha'l, chain-dotted connection, idle marking resistance YA in the primary finder to battery. Relay LK on operating in this circuit at armature lkl opens the circuit of the allotter magnet PAM before it has had time to operate and completes a circuit (or relay VR in series with the magnet which does notoperateinthese circumstances. Due tothe lowo! reiayLKanimmediateguard earthisextendedtothefinderseised. RelayVR upon operating completes a circuit from earth, armature or, (Fig. winding of relay VT, wiper RI, vertical wiper VW in normal position. left-hand winding of relay HA, oil-normal springsNl, armatmeshal and hbl,finder rotary magnetlultobattery; relayHAcperatesinthis circuit but relay VT and magnet RM are nonoperative. Armature or! extends battery by way of resistance YE and wiper R4 to the SM lead which is cross-connected at the I. D. F. to the positive conductor of the first selector, the A and B relays of which then operate to return a holding earth over the P conductor.

Relay HA upon operating, at armature ha! locks independently oi the oil-normal springs N8 and at armature ha. completes a self-interrupted driving circuit for the finder vertical magnet VM from earth, (Fig. 5) armatures vrl, vtl and n4, wiper R3, armature hat, rotary olinormal springs NR2, vertical magnetinterrupter springs om, magnet VM to battery. At armature hal earth is applied by way of springs N! to wiper R2 to further guard the finder against seizure by any other searching allotter. This earth shunts the left-hand winding of relay LK which however remains operated by way of its right-hand winding over wiper Rt to battery over the idle marking resistance YA.

By the operation of the magnet VM the finder is stepped vertically until the vertical wiper VW encounters battery on the marked contact in the vertical bank VB whereupon relay VT *quickly operates and opens the vertical magnet driving circuit at armature vti and also energises relay RS. Owing to the low resistance of relay VT the level is guarded against seizure by any other finder in the group. Relay R8 in the control set operates and locks by way 01 armatures or! and rsi and completes a holding circuit for relay ST at armatures rsl independent of the start earth. Armature vtl also opens the holding circuit of relay HA which thereupon releases, but the finder remains guarded by the earth extended from oilnormal springs N! which operate as soon as the switch steps vertically. On the release of relay HA the vertical magnet circuit is further disconnected at armature hat and at armature ha! a circuit is completed from earth on lead it by way of equipment which gives an alarm should the magnet remain energised for more than a predetermined period, test link T, rotary magnet interrupter springs rm, oil-normal springs NI, armatures ha! and hbl, magnet RM to battery and accordingly the wipers are rotated in search of the calling line.

when this is reached a circuit is completed to hold the finder rotary magnet in the operated position and thus arrest the motion. Assuming the marking is on the Mi bank, this extends as iollows: earth, armatures or. and vti, rectifier MRC in the conductive direction, wiper Rl, low resistance right-hand winding 01 relay HA, armature hbl, wiper Mi, armature 14 in the calling line circuit, vertical bank VB and wiper VW, rotary magnet interrupter springs m in the operated position, oil-normal springs NI, armatures no! and no, rotary magnet RM to battery. It the marking had been on the M2 bank, the jamming circuit would have been completed by way oi rectifier MRI) in the control set, wiper R! and the righthand low resistance winding of relay HB.

Assuming that the marking is on the MI bank, relay HA operates in series with the magnet. Armatures MI and hat extend the negative and positive conductors through to the first selector, armature not extends an earth over the finder Pl wiper to operate relay K in the calling line circuit. armature hat extends the meter lead Ii through to the outgoing P lead by way oi the rotary oil-normal springs NRI and NR2 and rectifier MRA, armature he! opens the magnet jamming circuit and completes a holding circuit for relay HA by way of rotary off-normal springs NR3 to the P conductor which is earthed from the selector while at armature hot this earth is extended over springs NR2, armature ha, wiper R3 and armature rsl to short-circuit and release relay ST in the control set. In the line circuit relay K upon operating disconnects relay L which thereupon releases and removes the vertical and rotary bank markings while at armatures II and k2 earth is connected to the subscriber's meter MTR in preparation for metering.

The start relay concerned which is assumed to be SA releases when relay L falls away assuming there are no further calls for lines connected to levels I and 2 and the armatures sal and so! remove the start and hold earths from the primary control set. Relay ST releases due to short-circuiting of its winding and at armature stl opens the holding circuit of relay LK which falls away and in turn releases relay VR. The holding circuit of relay RS is now opened at armature vrl while relay VT was previously released when relay L removed the vertical marking battery. The allotter PA now preselects a free finder in a manner to be described later and the control set becomes available for handling further calls.

It will be appreciated that if the calling party had been connected to the M2 bank, relay HB would have operated to switch through the second set of wipers. Relay H3 in addition to performing functions similar to those performed by relay HA. at armature hbl prepares a circuit for the common test buzzer connected to lead l5 so that by inserting a link into the test jack point T an attendant may readily determine which set of wipers is in use. Should two calling lines be encountered in the same position, relay HB has preference over relay HA since it opens the circuit of relay HA at armature hbl.

It may be explained here that in ordinary operation all switches are self-driven to decrease the hunting time. The drive is disconnected by a high speed low resistance relay switching to marking battery through a non-inductive resistance except in the case of the line finder rotary motion where magnet jamming is employed. Relays VT, Fig. 5, LK, Fig. 6, FK and DK, Fig. 8, come under this heading and are designed to operate in a minimum time to reduce the possibility of dual connections. Two such relays will not operate together in parallel to one marking battery and should one be already operated it will eifectively guard the outlet seized against another relay switching thereto.

Under normal light traffic conditions as soon as a control set is restored to normal it preselects a free finder under control of relay G. Earth from the operated springs N2 of the finder just set is extended by way of the chain-dotted connection, wiper R2, armature rfzl, left hand winding of relay LK, test link T, winding of relay G, armatures ofzl and .312, allotter magnet interrupter springs pam, armatures vrl, tbi and rs2, resistance YD to battery. Relay LK does not operate in this circuit but relay G operates and at armature gl energises the magnet PAM which opens the interrupter springs pam. Relay G thereupon releases and disconnects the magnet which then advances the wipers one step and again closes the interrupter springs. Should the allotter wipers now encounter an engaged finder, earth will again be extended over wiper R2 to re-operate relay G and the interaction will continue until a free finder is preselected. Should a further call originate before the completion or preselection, relay ST will re-operate and to reduce the finding time a circuit is completed at armatures st! and sti to self-drive the allotter in search of a free finder. As soon as the allotter encounters a free finder, relay LK operates to disconnect the drive and the circuit operation proceeds as described before.

As soon as all first choice finders in the group are taken into use, the idle marking battery which is normally extended from each free finder in the group through resistance YA, Fig. 3, chaindotted connections, armature hal, oil-normal springs N2, rectifier MR3 and chain-dotted connection to relay RFB which it is common to the group is removed. Relay RF'B thereupon releases and at armature rfbl, Fig. 4, operates relay RFZ over its upper winding. Armatures rfzl,

1722 and rfzl change over the test leads in the various primary control sets to wiper RI in order to prepare for testing last choice finders while armature rfzi completes the filament circuit of the valve VAA in preparation for testing last choice finders. rectifier MRB provided in each finder is connected up in such a sense as to prevent earth connected to the right-hand side of the rectifier from shunting the idle marking battery of other finders connected in common to relay RFB or valve VAA.

After completion of the setting of the last free first choice finder, the primary control set preselects a free last choice finder in conjunction with a free secondary control set SCI, Fig. 8. Should either the finder or its associated sec ondary control set be engaged an earth will be fed back on to the bank of wiper Rl to operate relay G which causes the allotter to step on until it encounters a free outlet.

When a further call is originated, relay ST operates and relay LK is then energised from earth, armature stl, left-hand winding of relay LK, armature rfzl. wiper RI, circle and dash connection, off-normal contacts N2, armature ha'l, lead SS, IDF cross connection to Fig. 8, armature fbi in the secondary start relay set SSI, winding of relay DS, rectifier MRG, armature fb3, to battery in parallel with a resistance marking battery YJ. Relay LK operates in this circuit and at armature lkl disconnects the allotter magnet circuit and operates relay VR which initiates the vertical hunting of the line finder. At armature or a marking battery is extended over wiper R4 to the SM lead and by the cross connection over the I. D. F. marks this finder in the secondary finder bank multiple, while armature vr3 connects the right-hand low resistance winding of relay LK by way of wiper R6 to the SS lead extending to the secondary control set thus holding relay LK over this winding after relay HA is operated in the finder. From this point the operation of the last choice primary finder proceeds as described for the first choice finder and when the calling line is located either relay HA or relay H3 is operated.

Relay DS in the secondary control set operates in series with relay LK and at armature dsl locks It will be understood that the up to the SS lead and at armature ds3 completes The ' search for the marked last choice primary finder.

Should the primary finder find the calling line before the secondary finder completes its finding operation, the primary control set will remain held until a signal is received from the secondary finder. The primary and secondary finders hunt simultaneously but since the secondary finders have no normal position and since two are set in operation together, one of the secondary finders will almost always find the primary finder before the latter has found the calling line.

As regards the detailed circuit operation, when relay DR operates, at armature drl it connects relay DK to the secondary allotter wiper SAI while at armatures dr2 and drl a self-interrupted driving circuit is completed for the secondary allotter magnet SAM. Since the allotter is preselecting by the interaction of relay GG and the magnet SAM, under light trafilc conditions its wipers will already be standing in engagement with a free secondary finder and relay DK will operate immediately over wiper SAI to the idle marking battery through resistance YG in the secondary finder SFI. Due to its low resistance relay DK provides an immediate guard on the finder seized. Armature dkl disconnects the driving circuit of the allotter and removes the short-circuit from relay FR which then operates in series with the magnet SAM which cannot now and also completes the circuit ofthe driving magnet SFM of the secondary finder seized. Relay FR. thereupon locks up at armature fr, at armature fr locks relay DR, at armature jrl extends earth by way of relay FK over wiper SAl to wiper SP4 of the secondary finder and at armature [11 extends earth over wiper SAI to the negative conductor of the secondary finder in order to operate the A relay in the selector connected to the secondary finder. This relay then operates relay B which returns an earth over the P conductor.

The driving magnet SFM of the secondary finder operates in a self-interrupted circuit to rotate the wipers in search of the primary line finder which is marked over the SM lead from the primary control set by battery through resistance YE. When this is found, relay PE in the secondary control set operates and at armature jkl disconnects the magnet drive circuit and extends earth over wiper SA! to operate relay SK in the secondary finder. Due to its low resistance the winding of relay FK provides an immediate guard on the primary finder seized. Thereupon armatures ski, ski and ski extend the negative, positive and P leads from the secondary finder wipers through to the selector, armature skl connects a further guard earth to wiper BM in order to short-circuit relay DK, armature ski locks relay SK to, the earthed P conductor while armature old connects up a,test buzzer over common lead 24 to a routine test jack. Relay DK now releases and prepares a circuit for shunting down relay FR.

If the primary finder reaches the calling line before the secondary finder completes its finding operation, there will be no hold earth extended over the P conductor to hold relay HA or relay HB operated in the normal manner and relay HA is therefore held operated in series with relay ST in the circuit: earth, armatures rs! and tb'l, winding of relay ST, armature rsl, wiper RI, armature lull or hbt, springs NR! and NR3, left-hand winding of relay HA or H13, armature Ml or hbi rotary magnet RM to battery. Relay ST is not yet short-circuited and the primary control set is therefore held until the secondary finder has completed its operation. As soon as the secondary finder finds the marked primary finder, earth will be extended over the P lead to maintain relay HA or KB in the primary finder and to short-circuit and release relay ST in the primary control set. On the release of relay ST the primay contol set becomes available for further common use and' the primary allotter preselects another free last choice finder.

As regards the associated secondary apparatus, when relay VR. in the primary control set releases after the release of relays ST and LK, the resistance marking battery is disconnected from the SM lead at armature or! and relay FK in the control set SCI is released. The start earth is already removed from the SS lead and relays DS and DR. release. Relay FK on releasing at armature lkl short-circuits relay FR. and during the release time of relay FR. an earth is extended over armatures flcl, dkl and {16 to energise the allotter magnet SAM so that when relay FR releases the allotter wipers are stepped to the next secondary finder. Should this finder be engaged earth will be extended over wiper SAI to operate relay GG which will interact with the magnet SAM to preselect a free secondary finder. Should the control group be again seized during this hunting operation, relays DS and DR will operate again and the relay drive for the magnet SAM will be changed to self drive to reduce the finding time to a minimum.

When the complete train of automatic switches is set up and the called subscriber replies, a booster battery metering impulse is applied to the P conductor and is extended over rotary off-normal spring NR2, rectifier MBA in the conductive direction, springs NRI, armature hat, wiper Ml, meter MTR, and armatures ll and R2 to earth so as to operate the meter in the calling subscriber's line circuit in the well-known manner. The rectifier MRA prevents relay HA or HR and the meter from holding operated in series when subsequently earth is removed from the P conductor when the calling subscriber restores his receiver.

It will be noticed that there are two terminals provided at the I. D. F. for connection to the meter MTR. and also that the necessity for inserting a separate rectifier in each individual meter circuit has been overcome by the arrangement according to the invention which only requires one in the line finder circuit. With the circuits shown by a simple alteration of jumpering at the I. D. F. the switch can be adapted to work in with any existing system of metering which may for example utilise booster battery operated meters each connected to earth by way of a rectifier; meters connected to battery with no rectifier in series which are operative only when booster potential is applied or meters operated over a fourth wire by the application of negative battery or by the application of earth potential. If therefore a line finder system of this type is installed in an existing exchange, no replacement of subscribers meters will be necessary and this represents a substantial economy.

At the end of the call when the calling subscriber replaces his receiver, earth is removed from the P conductor and in the primary finder relay HA or HB according to which has been operated is released and in turn releases relay K in the line'circuit. At armature M or M5 the operating circuit for the finder rotary magnet is again completed and the switch thereupon rotates to the 12th position in which the wipers are clear of the bank. In this position the supporting means for the wiper carriage are no longer operative and it falls by gravity assisted by a spring and then rotates to the normal position and thereupon the off-normal springs N3 open and disconnect the rotary magnet. The guarding earth extended from the operated springs N2 to the allotter bank during release is then removed and the finder becomes available for further use. Preferably while the switch wipers are falling and rotating back to normal the rotary magnet interrupter springs are mechanically prevented from opening so that the magnet remains energised and does not chatter.

If the call is completed through a secondary finder, when the earth is removed from the P conductor relay SK will also release and at armature skl restores the idle marking battery to mark the secondary finder free in the banks of the allotter. The wipers of the secondary finder remain in the position in which they were last used.

Should the calling subscriber restore his receiver before the finder has completed its vertical movement the level marking battery on the finder vertical and rotary bank multiples and the start earth to the primary control set would beremoved. Relays ST, LK and VR in the control set therefore release and armature or! disconnects the finder vertical magnet circuit and at the same time opens the holding circuit of relay HA in the finder. Relay HA upon .releasing completes a self driving circuit for the rotary magnet RM and the finder will therefore cut in on the level it has reached and rotate to the 12th position and then release If it had already commenced its rotary movement relays RS and VT in addition will be released and the switch will rotate clear of the bank and restore to normal as previously described.

Should the finder fail to find the vertical marking it will step to the 10th level and relay VT will be operated by the marking battery permanently connected to the 10th vertical bank contact. The wipers will then cut in, rotate to the 12th position and release. Since the start signal is maintained another finder will then be selected by the primary allotter and a search for the calling line will commence again.

As soon as the last idle last choice finder in the group is taken into use, battery which is extended normally from resistance YJ, Fig. 8, armature lbi, SS lead, armature hal, off-normal springs N2, rectifier MRB, circle and dash connection to the grid of the valve VAA is removed. The grid then becomes positive with respect to the filament which it will be remembered was heated when relay RFZ operated due to all first choice finders being busy The increase of anode current due to the positive potential on the grid results in the operation of relay OFB which at armature ofbl completes a circuit for relay OFZ which then operates in series with resistance YC as soon as relay VR releases after the final last choice finder has been fully set. Armature ofzl applies earth to lead 23 extending to the congestion meter, armatures ofzZ, jz3 and ofzl disconnect the start leads to the primary control sets while armatures ofz5, ofzi and 01.21 disconnect the circuits of relays G in the various primary control sets in order to prevent unnecessary hunting of allotters.

When a last choice finder again becomes free the grid of the valve VAA again becomes negative and relays OFB and OFZ release to open up access to the primary control sets. Similarly in case a first choice finder becomes free relay RFB re-operates and at armature rjbl short circuits relay OH and disconnects relay RFZ to open up the primary control sets for access to first choice finders.

If all the first and last choice finders cannot be accommodated on the banks of one allotting switch, an auxiliary allotter may be provided giving access to last choice finders and in these circumstances relay RFZ upon operating when all first choice finders are busy is arranged to transfer the wiper connections from the regular allotter to the auxiliary allotter. The transfer is delayed until the primary control set has completed the setting of the first choice finder last taken into use by arranging that the changeover relay required will not operate until relay VB is released and relay RFZ is operated. It is thus insured that there will be no interference with a finder which is in the process of being set.

If all secondary finders operated from a secondary control set become busy, the idle marking battery normally fed out from each finder in the group by way of resistances YF to a common lead extending to relay OB in the secondary start-relay set is disconnected at armature ski in the last secondary finder to be set. Relay 0B thereupon releases and at armature obl operates relay FB which at armatures jb6 and ,fb'l applies earth to mark the access points SS and ISS busy and at the same time disconnect the start relays DS in the secondary control sets SCI and S02. Should a secondary finder" be hunting at the time relay FB operates, relay FR will be already operated and at armature fr will maintain a circuit for relay DS until the call is switched through. Continuous rotation of the secondary allotters is prevented by disconnecting relay GG at armature fbl.

If the secondary group is seized over both routes of access when only one secondary finder is available, one of the callers will be unsuccessful as regards completing the connection over this secondary group, but as soon as the successful caller is switched through to the group selector, relay FR releases and at armature fr5 disconnects battery feed from relay DS since relay FB will now be operated. Consequently the primary finder of the unsuccessful caller is caused to release after which another primary finder connected to a diiferent secondary group will be taken into use and the call completed thereover.

In case a condition should arise when all secondary finders in the exchange are in use at the same time, the battery will be removed from all access points SS so that all valves VAA in the exchange operate their respective relays to connect up the congestion meters and prevent access to the last choice finders.

When a primary control set is taken into use, relay ST operates and as previously mentioned starts a delayed change-over set to give an alarm should the control set be held beyond the predetermined time; this is effected by armature stl which applies earth to lead ST extending to the delayed alarm set and this starts the machine which drives cams associated with the S and Z leads. After a short period earth is applied to the S lead from the machine and relay TA will operate over its upper winding from this lead and subsequently lock over its lower winding by way of armatures tal and stG. If the 7 control set is not released within the predeterminedperiod, earthisconnectedtothezleadto operate relay TB. This relay then locks to earth supplied to common lead II from the unoperated armature tbl of the TB relays in other primary control sets of the same group which are not faulty. The primary control set supervisory lamp H? is lit to indicate the fault condition while armatures to and tbl extend earth over common leads l1 and il to give general and classified alarms. Armature tb'l switches the control set out of service, releases relay ST and transfers the start circuit to another control set in the same group while armature tb! maintains a locking circuit for relay TA if relay VB is not operated and armature to. disconnects relay G to prevent rotation of the allotter.

If the control set is faulty and has failed to associate itself with a free primary finder, relay VR will not be operated. Relays TA and TB will therefore remain locked up until the fault release link FRL is momentarily removed when the fault is cleared by an attendant. If the primary control set has functioned properly and has become associated with a faulty finder relay LK. will be already operated and hence when relay TA operates relay LK will be locked up at armature tal. If the finder has failed to step vertically, armature orl will maintain relay HA operated and thus busy the faulty finder at armature hal. When relay TB operates, relay ST is released which then releases relay TA. This relay opens the holding circuit of relay LK at armature ml and relay LK in turn releases relay VR. Armatures tbl and to! thereupon maintain the circuit for relay HA over the allotter wiper RI. Should the finder step vertically but fail to rotate, relays LK, VR. and R8 will be disconnected when relays 8'1 and TA release, while the faulty finder will be busied by way of the operated off-normal springs Hi.

If the first group selector associated with a first choice finder is artificially busied for test purposes by the connection of earth to the release trunk conductor P, this earth will extend over of! normal springs Ni to operate relay HA. A burying earth will then be extended by way of armature M1, off-normal springs N2 and the chain-dotted connection to the R2 bank, and assuming that an allotter is standing on this finder the earth will be effective over wiper R2 to operate relay 0 and cause the allotter to preselect another finder. The rectifier MBA is non-conductive in this instance and prevents the earth on the P conductor from extending to the finder vertical mal et.

Ifthefirstselector associatedwitha secondary finderisartificiallybusied fortestpurposesby connection of earth to the P conductor, this earth will extend by way of armature ski and high resistanceYl'torelayOB. Itwillbeappreclated that owing to the presence of resistance YF relay 03 will remain held from idle marking battery through resistance Y! of other idle secondary findersofthesamegroup. 'lhebusyingearthis alsoextendedthrougharmatureskttobanksai earthbeingmli tothelowresistanceYG andcausingover-heating.

Keysareprovidedforroutiningfirstandlast choicsfindersandsecondaryfinderswhendm sired. To enable first choice finders to be routined, double-throw keys KRT (Fig. l) are pro vided one for each two levels and they simulate the start signal from any desired level. Ilor example if the routine key for level I is thrown, that is key KRT shown is operated to the left, marking battery is extended over springs KRTI to the 9th contact of the vertical bank VB and a finder is caused to hunt vertically until it encounters the marking battery when it will cut in on level i and rotate until it reaches the 11th position. The 11th position on the various levels is strapped in such a way that for example, on the first level a circuit will be completed over the Mi wiper to operate relay HA, on the second level there is a circuit for relay KB over the M2 wiper while on the third level there is a circuit for both relays HA and HB. Hence by operating the various routine keys KRT the operation of all the various circuits can be observed. When relay HA or HB operates the rotary magnet is held, the control set is released and since there is no circuit to hold either relay, the relay releases also. The finder will then step to the 12th position and restore to normal. The control set operates as usual to preselect another free finder and since the start signal is maintained the next free finder will be routined. All the first choice finders in the grou will therefore be routined in progression and the Operation will continue as described unless all first choice finders become engaged due to incident trafilc in which case the last choice finders will be brought into the test.

Should it be desired to routine last choice finders only, key KIT is thrown also. Key contacts KITI operate relay RH to transfer the test leads on the control sets to last choice finders while key contacts KIT! complete the valve filament circuit. If under these conditions all last choice finders become busy due to incident traflic, relay OFB associated with the valve VAA operates and serves to short-clrcuit relay RFZ. Further calls are therefore routed to first choice finders and the group is not closed until all first and last choice finders are ensaged.

A routine test of the valve VAA can be made by the use of the routine test key KVT which when thrown operates relay RFZ at springs KVTI, completes the valve filament circuit at pringsKvTl andconnectsearthtothegrid of the valve at springs KVTI thus causing relay OFB to operate if the valve is functioning correetly.

In order to routine secondary finders one double-throw key KB'I is provided per secondary startrelay set. Whenthekeyisthrowneither waycontacts KBTI markthelastoutletinthe secondary finder bank multiple which is reserved for routine purposu, and if for example, it is thrown upwards, contacts KB'I'I extend a start earthtotheupperseoondarycontrolsetwhile contacts K81! disconnect the circuit normally usedforsetting the secondaryfinderin operation fromthecontrolsetBCIonanordinaryca-ll. when the key is thrown downwarih the control ofthefindersby thelowersecondaryeontrolset canberoutined. Aleadisextuidedfromeach sldeofthekeyKsTlinthesewndarystai-tset sflitotheoppositecontrolset. 'lheseleadsare terminatedatthesbelfiackpolntsllandll andshouldacontrolsetberemoved,lackpdnts ltandllandli andflarearrangedtomske contact. Henoeifthe control setBCI is removedforexample,astartsignaioverlesdl88 will be extended through jack points 2| and 22 in contact to control set SCI.

Rectifiers MR1? and MRG in the secondary start set allow the use of one resistance battery feed for both the start relays in the two control sets SCI and SC2 so that a start signal received over the SS lead to the setSCl will not efiect the DS relay in the SC2 set and vice versa.

To test the stepping operation of the allotters for maintenance purposes the test link T is removed from the test points 25 and 26 (Fig. 6) and inserted between the test points 21 and 28 thereby disconnecting the preselecting circuit and enabling a self-interrupted driving circuit for the allotter magnet PAM to be completed by short-circuiting the test jack springs RT on the allotter in question. Similarly by the transfer of the test link T from jack points 29 and 30 to jack points 3| and 32 the secondary allotter may be operated from the test Jack RTI.

We claim:

1. In a telephone system, lines, a plurality of finder switches, a common equipment controlling the operation of any one of said finder switches to find a line when a call is initiated from said line, a driving magnet for each finder switch for operating it to seize said line, and means including a circuit extending into said common equipment for holding the magnet of a finder switch energized when the finder connects with the calling line.

2. In a telephone system, lines, a plurality of finder switches,a common equipment for controlling the operation of said finder switches to seize a line'when that line is calling, means associated with each line when a call is initiated therefrom for marking that line for seizure by a finder switch by means of a potential, and a circuit partially included in said common equipment for supplying said potential to mark said line as calling.

3. In atelephone system, lines, primary finders for connecting with said lines, groups of secondary finders associated with said primary finders, each group for connecting with certain of said primary finders, means responsive to a call on one of said lines for operating one of said primary finders to hunt for and find the calling line and for taking one of said secondary finders in the associated corresponding group into use, means for releasing both the operated primary finder and the associated secondary finder taken into use in case said secondary finder is unable to complete a connection, and means for taking another primary finder associated with a difierent secondary finder group into use to complete the call.

4. In a telephone system, lines, primary finders for connecting with said lines, groups of secondary finders, each such group having access to certain of said primary finders, and means responsive to a call on one of said lines for taking a primary finder into use only in case a simultaneous test indicates an idle primary finder and an available secondary finder in the group havingaccess to such idle primary finder.

5. In a telephone system, lines, a group of finders for connecting with calling lines, said finders arranged in normally used first choice finders and last choice finders used only in case all the first choice finders are in use, means for enabling a routine test of said last choice finders to be made by making said first choice finders artificially busy, and means responsive to all said last choice finders becoming busy for automatically rendering said artificially busied first choice finders available for handling calls initiated by said lines.

6. In a telephone system, lines, a plurality of finder switches each having primary and secondary movements to connect with a calling line, a common equipment for controlling the operation of said finder switches to seize a line when calling, means associated with each line when a call is initiated therefrom for marking that line for seizure by a finder switch by means of a potential, and a circuit partially included in said common equipment completed only after the primary movement of a finder switch for supplying said potential.

"7. In a telephone system, lines, a plurality of finder switches having vertical and rotary movements to find a calling line, a vertical test wiper in each finder switch, a common equipment for controlling the operation of said finder switches to find and seize a line when calling, means associated with each line when a call is initiated thereover for marking that line for seizure by a finder switch by means of a potential, and a circuit partially included in said common equipment completed over the vertical test wiper of a finder switch for supplying said potential.

8. In a telephone system, lines, a plurality of finder switches having primary and secondary movements to .find a calling line, primary and secondary motor magnets in each finder switch for operating said switches in their primary and secondary movements, means responsive to the initiation of a call on one of said lines for preparing a circuit to mark said line as calling, means also responsive to the initiation of said call for operating the magnets of one of said finder switches to hunt for and connect with the calling line, and means for completing said prepared marking circuit by connecting a potential through the winding of the secondary motor magnet of said finder switch in response to the operation of said finder switch.

9. In a telephone system, lines, a plurality of finder switches having primary and secondary movements to find a calling line, primary and secondary motor magnets in each switch for operating said switches in their primary and secondary movements, means responsive to a call on one of said lines for operating the magnets of one of said switches to hunt for and find the calling line, a marking circuit individual to said line prepared in response to a call on said line, means for connecting a potential through the winding of the secondary motor magnet of said finder switch to said prepared marking circuit to mark said line as calling, and contacts on said secondary motor magnet for maintaining said magnet energized in response to said switch finding said calling line by connection with said marking circuit.

10. In a telephone system, lines, a plurality of finders having primary and secondary movements to find a calling line, primary and secondary motor magnets in each finder for operating said finders in their primary and secondary movements, means responsive to a call on one of said lines for operating the magnets of one of said finders to hunt for and find the calling line, contacts on said secondary motor magnet included in a circuit for maintaining said magnet energized in response to said finder finding the calling line, and a switching relay in said finder operated in response to said finder finding the calling line for opening said maintaining circuit to release said secondary magnet.

- ll. In a telephone system, lines, primary finders for connecting with said lines, groups of secondary finders, a secondary control equipment for each secondary finder group for allotting idle secondary finders for use in completing calls when taken into use. each secondary control equipment associated with only certain 01 said primary finders, primary control equipment operated in response to a call on one of said lines for operatingoneoisaidprimaryfindersiofindthecalling line, a start lead ior each secondary control equipment, a potential normally connected to each start lead, a relay in said primary control equipment operated by the potential on the start lead when the associated secondary control equipment istakenintouseinresponsetothecallbythe calling line, means for disconnecting said potentialiromsaidstartleadtoreleasesaidrelayin case said associated secondary control equipment is unable to complete the call, and means responlive to the release oi said relay for reoperating said primary control equipment to release said operated primary finder and to take another primary finder associated with a difierent secondary control equipment into use to complete said call.

12. In a telephone system, lines, primary finders (or connecting with said lines, groups or secondary finders, each such group having access to certain of said primary finders, a test circuit for selecting an idle primary finder, a series of contacts included in said test circuit indicating that the primary finder is idle and that a secondary finder having access to such idle primary finder is available, and means responsive to a call on oneoi'saidlinesiortakingintouseanidle primary finder over said test circuit only in case a secondary finder is available.

13. In a telephone system, lines, a group of finders for connecting with calling lines, said finders arranged in normally used first choice finders and last choice finders used only in case all the first choice finders are in use, a relay, means for energizing said relay to make said first choice finders artificially busy to enable a routine test to be made oi said last choice finders, means responsive to all said last choice finders becoming busy for short-circuiting said relay to release the same, and means responsive to the release of said relay i or rendering said first choice finders available for handling calls initiated by said lines.

it. In a telephone system, lines, a plurality of finders for connection with calling ones of said lines, a booster battery circuit in each finder, a rectifier in each finder, means for including said rectifier in said circuit in response to the connection of the finder with a calling line, said rectifier preventing an improper operation of said finder during the operation of said finder.

15. In a telephone system, lines, a test contact and a cut-oi! relay for each line, a group of finders having access to said lines, means responsive to a call on one of said lines for operating one of said finders and for applying a marking potential to the calling line's test contact independent of the calling line's cut-ofi' relay to enable such finder to find the calling line, a wiper on said finder, and means responsive to the connection of said finder to said line for operating said cut-oi! relay over said wiper.

16. In a telephone-system, lines, a plurality of finders having access to said lines, means responsive to a call on one of said lines for operating one of said finders to hunt for and find the calling line, means associated with each line for preparing a marking circuit to mark such line as calling in response to the initiation of a call, means responsive to the operation of said finder for completing said marking circuit by connecting a potential to said prepared circuit to mark said line as calling, and means responsive to said finder finding said connected potential on said completed marking circuit for connecting said finder to the calling line.

17. In a telephone system, lines, primary finders for connecting with said lines, groups of secondary finders, aliotting means for each secondary finder group for aliotting idle secondary finders ior use in completing calls when taken into use, each aliotting. means associated with only certain of said primary finders. means responsive to a call'on one of said lines for operating one of said primary finders to find the calling line and for taking into use the associated secondary aliotting means, means ior releasing the operated primary finder in case said second ry aliotting means is unable to complete a connection, and means for taking another primary finder associated with a difierent secondary aliotting means into use to complete the call.

' 18. In a telephone system, lines, a plurality of finder switches for connecting with calling ones 0! said lines, a numerical switch directly associated with each finder switch and connected thereby to a calling line, means (or artificially busying a numerical switch for test purposes, a rectifier in each finder switch, said rectifier preventing operation of said finder switch whenever the switch associated therewith is made artficiall! busy.

REGINALD TAYLOR.

GEORGE THOMAS BAKER. 

